nixos/udev: Provide a better warning for FHS paths

We were trying to find FHS references in all of the rules found in
services.udev.packages. Unfortunately we're still fixing up paths in the
same derivation where we are checking those references, so for example
references to /sbin/modprobe were still printed to be needed to fixup
even though they were already fixed at the time.

So now we're printing a more helpful warning message which is also
conditional (before the warning message was printed regardless of
whether there are any rules that need fixup) and is based off the rules
that were already fixed up.

The new warning message not only contains the build-local rule files but
also the original files from other store paths and the FHS path
references that were still found.

With 8ecd3a5e

 reverted, we now get this:

/nix/store/...-udev-rules/63-md-raid-arrays.rules (originally from
 /nix/store/...-mdadm-3.3.4/lib/udev/rules.d/63-md-raid-arrays.rules)
 contains references to /usr/bin/readlink and /usr/bin/basename.

Which is now more accurate to what is not yet fixed and where it's
coming from.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>